It's been my experience that a macro shot, done well, will sell on microstock. It's like anything else. Got to be of a subject that is needed by designers, as well as something that is technically correct. Most of my recent macro's have been of technology subjects ... printed circuit boards, electronics and the like. The PC boards can be used as backgrounds due to the abstract patterns the circuits make on the board's surface. For electronics, what I've done is to get a software program called Helicon Focus. This enables me to take a series of exposures, each focused at slightly different points, and then blend them together to make an image that is completely in focus for whatever depth I need. One of my best sellers is of a PC board with several electronic components soldered on. By using the software, I was able to shoot the board in macro mode at about a 15 degree angle up from the horizon and get the entire board and all components in sharp focus from front to back, a distance of about 2-3". Made a great 3D effect.
I've also shot nature macro's ... insects and the like. But they don't seem to do as well as other topics. I know there is a market for these subjects. It just doesn't seem to be as large as for other things.